What should I do about my increasing doubts regarding my brother's wife and her potential affiliation with people of innovation (Ahl al-Bid'ah), especially after I noticed my brother's ideas increasingly inclining towards them, knowing that he does not wish to divorce her due to her good character, her treatment, and their children together?

The ruling on marriage to someone with differing beliefs has been previously explained, detailing the distinction between disbelief-inducing innovation (bid'ah mukaffirah) and non-disbelief-inducing innovation. Based on this, the ruling on the marriage is determined. In the event that the marriage is deemed invalid, the children are attributed to the husband if he believed his marriage to be valid. We recommend advising both of them and clarifying the falsehoods of the sect to them.